Direct viewing moving target indicator cathode-ray storage tube



Feb. 10, 1959 M. E. CROST 2,873,393

DIRECT VIEWING MOVING TARGET INDICATOR CATHOBE RAY STORAGE TUBE Filed Jan. 4, 195a INVENTOR, MUNSEY 5 CROSZ DIRECT VIEWING Movrraa TARGET INDICATOR 'CATHODE RAY STfiRAGE TUBE Munsey E. 'Cl'OSt, Loch Arbour, N. 3., assignor to the J United Statesof America as represented by the Secretary of the Army United This invention relates to electron discharge devices of 'tbetype known as cathode-ray storagetubesrand, more particularly, to improved means forseparating and displaying radar information relating to moving targets from simultaneously occurring information relating to stationary targets in the. raw mixed video return signal.

In-the prior art, the above results are accomplished by theme oia combination of devices. Either some type of acoustic delay'line or comparison type cathode-ray storage tube is used in order to isolate the movingtarget information from the remainder of the raw mixed video return signal. The return signal from a given radar pulseywhich' is scanned according to a definite program,

is compared with the return signal from a previous radar pulse. This involves a process of cancellation. Those signals which occur with the same amplitude and position atthe same interval following the initial signal in successive scans are cancelled. However, those signals which appear during successive scans with a varying amplitude, or at different times, are transmitted and appear in theoutput of the device to indicate moving targets. The output appears only as an electrical signal -as.a function of time. In order to display this output,

complex associated circuitry in addition to a device such as a viewing cathodeuay tube is required.

The principal object of the present invention is to provide in a single device both means for extractingmoving target information from the raw mixed video signal and means for displaying this informationon aphosphor screen.

Affurther object of the invention is to combine the 'function of a cathode-ray tube of the signal cancellation '.type and a cathode-ray tube of the display type in a single apparatus. r

Other objects and advantages will'beapp'arent from the following detailed description and the accompanying drawing; wherein:

,Figsl is a cross-section of one embodiment of a direct viewing moving target indicator cathode-raystorage tube constructed .in accordance with the principles of the invention; and

Fig. 2 is .ment of a directviewing movingtarget indicator: cathoderay storage tube constructed in accordance with the principles of the invention.

In, a first embodiment of 'a tube constructed in ac-' cordance with the principles of the invention, referring to Fig. 1, the tube comprises an evacuated envelope 3,

symmetrical about an axis 5 and composed of a suitable 'material, such as glass. In a tubular neck at one end of "the envelope -3, is situated an electron gun 4 -centered about the axis of symmetry 5. The envelope '3 has a larger diameter'at the other end and terminates in an enclosure "for a fluorescent screen 6 consisting of an electron sensitive phosphordep osited upon asemi-transparent, electrically conductive coating. The I screen 6 is connected electrically tothe-outside by a metallic conductor a cross-sectional view of another embodi sealed'rthrough'tbe envelope 3 in a conventional manner. i A

.control :electrode '10 surrounding the cathode.

2 ,873,398 Rater-tied Feb. Mi, 1 359 The electron gun 4 consists of the usual indirectly heated cathode .8 (the heater not being shown) and fie 'means for Jcontrolling'the intensity of the electron beam emission from thecathodasuch as a centrally apertured A relatively short distance from the control electrode '10, in the direction towards the screen d, is a centrally apere -:tured accelerating electrode 12. The apertures of both electrodes are aligned along axis 5.

A relatively large distancefrom the acceleratingelectrode 12, in the direction towards the screen 6, is an electron shield 1 grid '14 extending substantially perpendicularly'to the tube axis 5. Grid14 is constructed to have the lowest possible secondary electron emission ratio while also having -a large electron transparency, preferably of the'order .ot'90%. There is a first conductive element 16, formed by'a conductive coatingysuch as metal, extending on *the inner walls of the envelope 3 from the neighborhood of theaccelerating electrode 12 to the neighborhood of the electron shield grid 14. The combination of the-conductive coating 16, with a means forelectrically connecting it to the outside of the envelope 3 such as connecting lead 18 sealed through the envelope 3, and 'mea'ns'for maintaining it at any required potential, such as a battery 20 connected to the electrode 18, is well known inthe storage tube art.

Ajrelatively short distance from grid 14, in the direction of the screen 6, is-a cancellation -grid 22 extending substantiallyparallel to grid 14. .Grid 22 has a base 24 of electrically eonductivefmaterial and having a relatively large .number of apertures .per unit area, to provide a relatively'large percentage of electron transparency, preferably .of'the order of 50%. Both grids .14 and.24 are 'formed of fine meshed bars and have been purposely shown out of proportion in the drawings in order to show more clearly "the structure thereof. The surface of the deposited by a suitable method, such as vacuum evaporation, to cover only :the bars and not the holes of the 'base 24. The surface of this insulator .layer 26 facing the .electrongun carries a plurality of minute conductive spots 28, each of which is composed of "a material such as metallic beryllium, which is an electrical conductor and is weakly photoelectrically emissive, i. e., it will emit electrons when illuminated by radiation. The plurality of spots 28 are electrically insulated from each other and form with the base 24 and the intermediate dielectric layer 26 alike plnralityot minute condensers.

Located in the region :;between the electron gun 4 and the electron shield grid 14 are conventional orthicontype electron beam:deflec'tion means and complementary electron beam focusing means. Said deflection means includes electromagnetic deflection coils30 encircling :the envelope 3, although electrostatic deflection means may beused. Said ,Lfocusing means includes an electromagnetic focusing solenoid B t-encircling the electromagnetic deflection coils. The two. means above'mentioned should have such characteristics that the electron beam directed to any point on the surface of the dielectric layer 26 will arrive .in a direction. substantially perpendicular to that surface. In addition, theremay be additional means for aligning the electron beam, such'as electromagnetic alignment coils :35 situated within the focusing solenoid 34.

YAmeans for maintaining a'constant weak electron discharge of the minute photoelectrically active conductive spots 28 is providedby a source of controllable radiation, e. g., ultraviole'tra'diation, which may bein the form of a ringshaped lig'htsourcefifi surrounding the envelope 3 soas to illuminate the minute conducperiod.

'tained near cathode potential.

asvases tive spots 28. Light shields (not shown) should be placed around the illuminating source 36 so as to confine the illumination from s urce 36 to the region between 14 and 22.

maintaining the grids 10,, 12, 14 and 22 and the screen 6 at the required potentials.- This maybe accomplished .by sources of voltage connected tothe respective grids by means of terminals sealed through the glass envelope in accordance with conventional practice;

The mode of operation of the tube shown in Fig. .l'is as follows: The electron gun 4emits an electron beam toward the screen 6. The conductive coating 16 on the wall of the envelope acts'as an accelerating electrode. The deflection coils 30 actto create fields which sweep the electron beam in mutually perpendicular directions at arbitrary relative rates resulting in any required. scanning pattern on the cancellation grid 22. V I

The electron shield grid 14 operates at cathode potential, which arbitrarily may be any suitable referencc voltage, e. g., zero volts. The very high electron transpar'ency of this grid prevents excessive attenuation of the electron beam traversing it. V g

The electron beam is accelerated by the voltage of the battery 20, which may be of the'order of 500 volts, applied to the conductive coating 16. The operation of the electromagnetic deflection coils 30 combined with the system of voltages consisting of thisapproximately 500 volt accelerating potential, the approximately zero volts of the electron shield grid 14, the approximately zero volts of the base 24 of the cancellation grid 22, and the fields of the electromagnetic focusingts'olenoid 34, is a standard method in the art for producing anorthicon type of scan of the electron beam. This scan can assume any desired pattern, such as a PPI (plan-'pos'itionindication) or a rectilinear pattern, with the electron beam having substantially perpendicular incidence upon-both grids 14 and 22. V,

The focusing solenoid is adjusted to produce focusing'of the electron beam upon the surface of theifcanc'ellation grid 22. In the absence of modulation, the electron beam from the electron gun 4 is operated at constant current. except for a blanking signal usually applied to. the control electrode to cut off the electron beam during the fly-back or non-operational parts of the scanning The bias potential on the base 24 of grid 22is main- Unidirectional modulation corresponding to the radar video'return signal is applied to the base 24 of grid 22 as a positive potential.

A condenser is formed eifectively by eachof the conducting spots 28 and the base 24 ofthe cancellation grid 22. At the beginning of the scanning operation both the base 24 and all the conducting spots 28 are postulated to be at cathode potential. With no positive potential signal, applied to the base 24, all the conducting spots 28 will remain at the potentialof the base, i. e., substantially at cathode potential.

In the tube along the path of the electron beam from the cathode to the cancellation grid 22, the electron beam is acted on by a series of accelerating electrodes whose system of potentials constitute a' conservative potential field. In such a case, the speed of the electron at points of the same potential is the same. Therefore,.the electron beam has the same speed at the cancellation grid, which is at cathode potential, as it hadat the cathode, and no electrons will be incident. upon or penetrate the cancellation grid, Therefore, with no positive signal'applied to the base 24 of the cancellation-grid, no electrons will reach this grid or penetrate it to screen 6. .Q'Ihe application of the positive radar signalf potential to the base 24 causes the conducting spots 28 to assume the same positive potential by capacitive action. At any given point in the scan, the electron beam will tend to impinge on the cancellation grid 22 when a positive signal is applied to the base 24. About half the electrons in the electron beam incident upon the cancellation grid will impinge upon the insulating layer 26 and the conductive spots 28.

At the beginning of the scanning operation, those electrons of the electron beam impinging upon the spaces in the cancellation grid 22 upon application of the positive potential signal will penetrate the cancellation grid. However, this penetration is only a transient state occurring at the outset of the scanning operation since, as will be hereinafter described, a negative potential barrier of electrons accumulated on the conducting spots in a few scanning periods will prevent penetration of the electron beam after these few scanning periods.

As is well known in the art, since the base 24 of the cancellation grid 22 is kept near cathode potential, the secondary emission ratio of each of the conducting spots 28 of the cancellation grid 22 can be made less than one. Therefore, with the same positive radar signal voltage applied to the base 24 at the same relative time in successive scans, the collision of the electron beam with the same conducting spot will effect a charging of this spot negatively from the on'ginal positive potential towards cathode potential, since each of the conducting spots 28 upon which the constant current electron beam impinges emits secondary electrons which are lesser in number than the primary electrons from the impinging electron beam.

Therefore, those spots upon which'the electron beam impinges will charge negatively, until an equilibrium potential condition is reached, when the spot of the insulating layer is at cathode potential at the time the positive potential is being applied to the base 24. Consequently, the negative potential barrier produced by the electrons accumulated on the conducting spot varies directly as the positive potential signal applied to the cancellation grid base at the time the beam is on the particular spot. This results in an intensity modulation of that part of the electron beam penetrating cancellation grid 22. v In the neighborhood of those portions of the cancellation grid 22 which reach cathode potential, electron transmission through the cancellation grid will substantially cease,.since this is a conservative potential field, and no accelerating potential for the electrons will remain between the cathode and the cancellation grid as herein above described. Consequently, there also will be no luminescence, upon the corresponding portions of the fluorescent screen 6. The control characteristics and bias voltage. of the base 24 of thecancellation grid 22 are chosen to ensure this efect. Electrons which approach those portions of the cancellation grid which have, reached cathode potential or a more negative potential wilLbe repelled and will form an electron space cloud in the zone betweentheelectron shield gridp14 and the cancellation grid. This electron space cloud will subsequently reach an equilibrium size determined by balance between the current collected by the conductive coating 16 on the inner wall of the envelope. 3 and the current arriving in the electron beam.

Assuming that the unidirectional modulation corresponding to-the radar video return signal applied to the base 24.0f the cancellation grid is periodic and has the same value at .the corresponding time in successive scans,

which is the case when the return signal corresponds to a stationary target, the points in the scan on the cancellation grid :22 will accumulate electrons to ultimately reach cathode potential while the electron beam is directed toward those points. This will cause electron transmission through the'cancellation grid 22 to cease, as .hereinabove explained, in those portions, of the cancellation grid which are in the immediate neighborhood of these .points. :r; Assuming the unidirectional modulation potential,ap-

plied to the base 24 has awalue more positive than the equilibrium potential at the corresponding time in .successive scans, which is the case when thereturn signal corresponds to a moving target, then corresponding points in the scan on the cancellation grid 22 will be above cathode potential, and electrons will be transmitted through these points on the cancellation grid 22 to cause illuminalion of the corresponding portions of the fluorescent screen 6 until these points on grid 22 also charge to a potential near cathode potential, because of electrons captured from the electron beam. When the moving target signals, as represented by the more positive potential disappear or move to other positions in the scan, these points'which hadbeen charged to near cathode potential when the moving target signals were present will assume a potential negative with respect to cathode in the absence ,of the moving target signal in succeeding scans. Not only .doesthis prevent the electron beam from penetrating the cancellation grid 22 in succeeding scans at these points, but it also prevents the beam from penetrating the cancellation grid at those points with later moving target signals of less positive potential.

An important feature of this invention is to provide means to discharge these points which have a potential negative with respect to cathode in the absence of moving target signals back to equilibrium potential. The source of radiation 36 does this by causing slow photoelectrical discharge of the conducting spots 23. The effect of this slow photoelectrical discharge of the conducting spots in those portions of the scan on the cancellation grid where moving target signals were not present, and which were at equilibrium potential, will be overcome as each subsequent scan replaces the electrons lost by the photoelectrical discharge; v

, However, in those parts of the scan on the cancellation grid where moving target signals had been present and which subsequently had disappeared or moved to other locations, the combination of the efiect of this slow photoelectrical discharge of the conducting spots 28 and the structure and function except that screen 6 has been placed farther along the tube axis from grid 22. To avoid repetition of the description this identity has beenindicated by adding the'quantity 1% to the reference numerals used in Fig. 1 to produce the reference numerals designating these elements in Fig. 2. The second em bodiment includes additional structure located between the cancellation grid 22 andthe fluorescent screen 6.

This additional structure includes a means to "produce a flood of electrons of constant current density directed towards said screen throughout the region between the cancellation grid and the fluorescent screen. Said means should not interfere with the normal status of the-electron beam penetrating the cancellation grid. Although such means can take form of an auxiliary hot cathode, the preferred form, as shown in Fig. 2, includes a photoelectric grid 138, spaced a relatively short distance from cancellation grid 122, in combination with a second radiation source 146, and a means, such as a grid 144, for accelerating the electrons toward the screen 106. Photoelectric grid 138 is perpendicular to the tube axis and has the highest possible electron transparency, prefer ably of .theord'er of 90%. The side of grid 138 facing the fluorescentpsc'reen has a photoemissive surfa'cel140 which readily emits electrons when illuminated by radiation. The 'secondradiation source 146, which may be potential.

in the form of a ring encircling the tube envelope, illuminate's the emissive surface 1% so as to 'cause emission therefrom. As in the case of light 36 inFig. 1, light sources 136 and 146 should be shielded sons to confine the rays therefrom to the regions they are designed to illuminate. Thus, the light from source 138 is to bedirected only-to the region between electrodes 114 and 122, while the light from source 146 will be confined tothe region ofphotoemissive surface 14.

An example of means to accelerate the photoelectrons thus liberated includes a second conductive ring 148, which is transparent and may be in the formof a coating on the walls of the tube, and a conventional accelerating mesh 144. The second conductive coating extends'on the inner walls of thcenvelope 102 from the neighborhood of the accelerating mesh 144 to theneighborhood of the photoelectric grid 138. This second conductive coating is transparent to the second radiation source 146 and is-electrically connected to the outside of thecnvclope 163 while being maintained at any required potential by a suitable means such as a source of variable voltagelStl. The conventional accelerating mesh 144 extending substantially perpendicularly to the tube axis 105 and relatively close to the storage grid is positioned between photoelectric grid 138 and the storage grid 145.

At a relatively long distance from the photoelectric grid 138 in a direction of the fluorescent screen 106 is a storage grid 145 formed on a base 141. Base 141 is a line conducting mesh extending perpendicularly to the tube axis 105 and having its surface which faces the electron gun 105 faced with an electrically insulating layer 142. i Y

Storage grid'145 issituated a relatively short distance before fluorescent screen 106. Conventional means are provided for focusing electrons penetrating the cancellation grid and traversing the region between the cancella tionfgrid 122 and the storage grid, such as a second electromagnetic focusing solenoid 147 extending in. this region outside the envelope 103. i

Conventional means (not shown) are provided for maintaining each of the above described meshes or grids 116, .112, 114, 122, 138, 144 and 145 at any required No two of the above described meshes or grids 114, 122, 138, 144 and 145 are situated so that their respective mesh bars are oriented in parallel directions. In other words the bars of one must be at angle to the bars of all the others. 1

It is to be understood that the spacing between elements 114 and 122 will be the same as the spacing between elements 14 and 22 in Fig. 1. In Fig. 2 the spacing between the elements is shown compressed to a facilitate the drawing. 7

Up to the point where the electrons penetrate vcancellation grid 122, the device in Fig. 2 operates in the same manner as the tube in Fig. 1.

Most of the electrons penetrating cancellation grid 122 will penetrate the fine mesh 138, because of its high electron transparency, and approach storage grid 145. The

storage grid is maintained at a potential suchthat the secondary electron emission coeflicient of the material of which the insulating layer 142 is composed is greater than unity for theelectrons penetrating the cancellation grid and reaching the insulating layer.

T he photoelectric grid 138 is maintained within a few voltsof the potential of the storage grid 145, the exact voltage depending on theelectron transmission characteristics of the storage grid Those electrons which penetrate cancellation grid 122 will be focused upon the storage grid by the second focusing solenoid 147 and cause the storage upon this grid of signals which are characteristic of the intensity and location on the storage grid of the originalelectrons, as follows: Itfis assumed that the insulating coating 142 is at first charged negatively with respect to the backing mesh-of storage grid- 145 by a voltage suificient'to prevent transmission senses of electrons from photoelectric grid 138. The greater the number of electrons upon storage grid 145 the greater sulating coating are collected by accelerating mesh 144. Thus the points in the screen on storage grid 145 bearing moving target information will be relatively close to the potential of the photoelectric grid 138 and allow electrons to penetrate, while those points in the scan in the storage grid bearing stationary target information will remain negative with respect to the photoelectric grid 138 and will obstruct the passage of electrons.

.The recharging negatively of the insulating coating 142 can be accomplished by means well known in the art such as the temporary application of a positive potential to the storage grid base 141, thus raising the potential of .the insulating layer above the potential of the photoelectric grid 138 by a voltage less than the first crossover voltage of the insulating coating 141 which will cause the insulating layer to rise in voltage by a like amount by capacitive action. Electrons will be collected on those portions of the insulating layer which had been discharged to less than the potential of the photoelectric grid 138 with positive signals present until their potentials are equal to that of photoelectric grid. Removal of thetemporary positive voltage pulse from grid base 141 will then cause potential of insulating coating 142 to become more negative than voltage of photoelectric grid 138 by alike amount.

The electrons which penetrate storage gridl45 come from the cathode means herein above described as follows: The photoelectric grid 138 and the accelerating mesh 144 are relatively far from one another leaving sufiicient space for the unobstructed illumination of the --photoemissive surface 140 by the source of radiation 136. Photoelectron emission at the photoemissive sur face 140 occurs as a result of this illumination. The resulting photoelectrons are accelerated by the action of the second electron coating and accelerating mesh 144. A more nearlyuniform accelerating field for these photoelectrons is provided by the accelerating mesh 144. The 'distance between the photoelectric grid 138 and the storage grid 145 is determined by the condition that photoelectrons are not to be focused on the insulating coating 142 of the storage grid in order to obtain a constant current density and uniformly diffuse flood of electrons.

Since the storage grid is maintained at a potential negative with respect to the photoelectric grid, the photoelectrons do not cause the storage of any signals upon the storage grid 145. However, since the fluorescent screen 106 is maintained at a potential several thousand volts higher than that of storage grid, at those points on the storage grid 145 bearing moving target information, as where there is a less negative potential, these photoelectrons can be acted upon through the storage grid 145 by the high electric field created by the fiuorescent screen 106 and can penetrate storage grid 145. It is thus apparent that the action of this photoelectric cathode and storage grid system allows the use of an electron current greater than that inherent in the orig inal electron beam when it penetrates cancellation grid 122. Because of the high potential of the fluorescent screen 106 and the continuous high photoelectric cur rent photoelectrons which have penetrated, storage grid 145 will cause greater fluorescence on the fluorescent screen in the positions appropriate to the corresponding points in the scan bearing moving target information.

To allow for the recharging of the insulating coating 142 so that moving target information will not be retained indefinitely in regions of the storage grid, there are applied to the storage grid. base 141ishort positive voltage pulsessutficient to raise .;tem'porarily by capacitive action the potential of insulating coating 142 to a value exceeding the potential of the conducting mesh 138. Electrons will be attracted to the insulating layer v142 during the time of the application of these pulses, and gradually these moving target information regions on storage grid 145 will attain a potential sufliciently negative to prevent photoelectron penetration.

Since no. two of any of the plurality of meshes or grids have their mesh bars aligned or pointing in the same direction, the interference beat note in the output known as the Moir pattern is avoided.

The invention described herein may be manufactured and used by or for the Government for governmental purposes, without the payment of any royalty thereon.

What is claimed is:

1. A cathode-ray storage tube indicator comprising an evacuated envelope having therein linearly arranged in the order named, an electron gun, a first metallic mesh grid, a second grid and a fluorescent screen, for displaying electrons traveling from said electron gun through said grids the distances from said grids to said electron gun being larger than their distances from said screen, the distance between said grids being small relative to their distances to said electron gun, means to align and accelerate the electrons from said gun toward said screen, said second grid comprising a metallic mesh base having on the side thereof which faces said electron gun a plurality of photoelectrically emissive conductive spots insulated from each other and from said base, said spots having a secondary emission ratio which is less than unity whereby the incidence of electrons thereon will tend to charge them negatively and reduce the flow of electrons to said screen, and means to apply upon the base of said secondgrid an electrical signal to be observed upon said'screen.

2. A cathode-ray storage tube indicator comprising an evacuated envelope having therein linearly arranged in the order named, an electron gun, an electron shield grid, a cancellation grid and a fluorescent screen, for indicating electrons traveling from said electron gun through said grids the distances from said grids to said electron gun being large compared to their distances from said screen, the distance between said grids being small relatively to their distances to said electron gun, means to accelerate the electrons from said gun toward said screen, means to align and focus said electrons upon the surface of said cancellation grid facing said gun, and means to scan said beam across said surface, said cancellation grid comprising a metallic mesh base having an insulator coating on the side facing said gun, a plurality of photoelectrically emissive conductive spots on said coating, said spots being insulated from each other, said spots having a secondary emission'ratio which is less than unity whereby the incidence of electrons thereon will tend to charge them negatively and reduce the flow of electrons to said screen, both of said grids having a relatively large electron transparency, a source of light to illuminate said spots to render them slightly photo-emissive, and means to impress upon the base of said cancellation grid an electrical signal to be indicated on said screen.

3. A cathode-ray storage tube indicator comprising an evacuated envelope having therein'linearly arranged in i the order named, an electron gun, an electron shield align and focus said cancellation grid facing said gun, and means to scan said beam across said surface, said cancellation grid comprising a'metallic base having an insulator coating -on the side facing said gun, a plurality of, photoelectrica'b ly emissive conductive spots on said coating, said spots s ular-cs being insulated from each other, :said spots having a ission ratio which isless than unity whereby the incidence of electrons thereon will tend to charge them negatively and reducethg: flow of electrons to said screen, both of saidgridshaving. a relatively large electron transparency, the electron transparency of said electron shield grid being larger than thatof said cancellaati'on grid, means to illuminate said spots to render them photo-emissive, and means to apply to the base of said cancellation grid 2. potential to be indicated on said screen.

4. A cathode-ray storage tube indicator comprising an evacuated envelope having therein linearly arranged in the order named, an electron gun including a cathode for emitting electrons and means for controlling the intensity of emission of said electrons from said cathode, a first grid, a second grid having a secondary electron emission ratio which is less than unity, a fluorescent screen, the distances from said grids to said electron gun being large compared to their distances from said screen, the distance betweensaid grids being small relative to their distances to said electron gun, means to accelerate the electrons from said gun toward said screen, means to align and focus'said electrons in a beam upon the surface of said second grid facing said gun, and means to scan said beam across said surface, said second grid comprising a metallic base having on the side thereof which faces said gun a plurality of photoelectrically emissive conductive spots insulated from each other and from said base, both of said grids being adapted to be.

maintained near cathode potential and having a relatively large electron transparency, the electron transparency of the first grid being'larger than that of said second grid, said spots having a secondary emission ratio which is less than unity whereby the incidence of electrons thereon will tend to charge them negatively and reduce the flow of electrons to said screen, means to illuminate 'said spots to render them photo-emissive, and means to apply to the base of said second grid an electrical signal to be indicated on said screen.

5. A cathode-ray storage tube indicator comprising an evacuated envelope having therein linearly arranged in' the order named, an electron gun including a cathode for emitting electrons and means for controlling the intensity of emission of said electrons from said cathode, a first grid having a minimum secondary electron emistensifying and focusing ;on said screen electrons traversing said cancellation grid.

6. A cathode-ray storage tube indicator as set forth in claim -5, wherein said last named means includes means to produce a flood-of electrons having a constant current density and dircctedtowards said fluorescent screen throughout'the interior of said tube, and a storage grid situatedbetween said-means and said fluorescent screen, said storage grid comprising-a metallic base havingan insulator coating on the side facing said electron gun.

7. A cathode-ray storage tube indicator as set forth in claim 5, wherein the last named means includes a photoelectric grid, a storage grid situated between said photoelectric grid and said fluorescent screen, said photoelectric grid comprising a metallic base having a photoelectrically emissive coating on the side facing the tinorescent screen and said, storage grid comprising a metallic base having an insulator coating on the side facing said electron gun.

8. A cathode-ray storagetube indicator as set forth in claim 5, wherein the last named means includes a photoelectric grid, a storage grid situated between said'photoelectric grid and said fluorescent screen, an accelerating mesh situated between said storage grid and said photoelectric grid, said photoelectric grid comprising a metallic base having a photoelectrically emissive coating on the side facing the fluorescent screen, said storage grid comprising a metallic base having an insulator coating on the side facing said electron gun, means to illuminate said photoelectrically emissive coating to cause emission of photoelectrons, and means to accelerate said photoelectrons uniformly in the direction of said fluorescent screen.

9. A cathode-ray storage tube indicator as set forth in claim 5, wherein the last named means includes a photoelectric grid having a relatively large electron transparency, a storage grid situated between said photoelectric grid and said fluorescent screen, an accelerating mesh situated between said storage grid and said photoelectric grid, said photoelectric grid comprising a metallic base having a photoelectrically emissive coating on the side facing the'fluorescent screen, said storage grid comprising a metallic base having an insulator coating on r the side facing said electron gun, means to focus upon sion ratio, a second grid havinga higher secondary elec- I tron emission ratio which is less than unity, and a fluorescent screen, the distances from said grids to said electron gun being large compared to their distances from said screen, the distance between said grids being small relative to their distances to said electron gun, means to accelerate the electrons from said gun toward said screen, means to align and focus said electrons upon the surface of said second grid facing said gun, and means to scan said beam across said surface, said second grid comprising a metallic base having an insulator coating on the side facing said gun, a plurality of photoelectrically emissive conductive spots on said coating, said spots being insulated from each other, both of said grids being adapted to be maintained near cathode potential and having a relatively large electron transparency, the electron transparency of the first grid being larger than that of said second grid, saidspots having a secondary emission ratio of less than unity, whereby the incidence of electrons thereon from said electron gun will tend to charge said second grid negatively and cut 01f the flow of electrons to said screen, means to illuminate said spots to render them slightly photo-emissive and thereby tend to dissipate the negative charge thereon, means to apply to the base of said second grid a positive-going repetitive electrical signal to be indicated upon said fluorescent screen, and means situated between said cancellation grid and saidfluorescent screen for insaid storage grid said electrons traversing said cancellation grid, means to illuminate said photoelectricaily emissive coating to cause emission of photoelectrons, cans to accelerate said photoelectrons uniformly in the direction of said fluorescent screen, and means for impressing any required sequence of potentials upon said storage grid metallic base. 7 V

10. A cathode ray tube comprising an electron gun including a cathode for emitting electrons and means for controlling the intensity of emission of said electrons from said cathode, a fluorescent anode, a grid interposed between said electron gun and anode and comprising a metallic mesh having a dielectric coating on the side thereof facing said electron gun, and means to apply a positive-going signal to said mesh, said grid being adapted to be biased at a potential near that of the cathode, the secondary emission ratio of said grid being less than unity whereby, upon impression of said positive-going signal on said mesh, electrons from said gun will strike said grid causing a negative charge to develop in the region (Referenc s on following page) 31 12 References Cited in the file Of this patent 3 FOREIGN PATENTS UNITED STATES PATENTS 1,007,587 7 France Feb. 6, 1952 2,021,907 Zworykin Nov. 26, 1935 I 2,322,361 Iams June 22, 1943 5 7 OTHER REFERENCES 2,540,632 Rose Feb. 6, 1951 2 'Kn01l: Storage Tubes, John Wiley and Sons, Inc., 2,654,853 Weimer Oct. 6, 1953 1952, pages 79 and 82.

2,740,918 James-ct a1. Apr. 3, 1956 2,824,248 Szegho Feb. 18, 1958 

